Open iMessage, under the message settings, a popup message will appear saying your carrier may charge for SMS messages and select “cancel”. iMessage activates itself after you have clicked cancel. But this would not fully activate itself. Instead, you activated iMessage via e-mail, so you can only send messages with your e-mail address.

Try restarting your device. To do this simply, press and hold the Sleep/Wake button until the red “slide to power off” slider appears, and then slide the slider; and then Press and hold the Sleep/Wake to turn on.

Turn off iMessage and FaceTime. Go to Settings > Phone. Check if your number is reflected in My Number and restart your device.

Settings -> General -> Restrictions > Accounts and make sure that you selected “Allow Changes” and enable the FaceTime to work.